1. Field of the Invention
The present invention relates to semiconductor devices and methods for manufacturing the same.
2. Description of the Related Art
In recent years, semiconductor devices have been actively manufactured, in which thin film transistors (TFTs) are formed over a substrate having an insulating surface, such as a glass substrate, and the thin film transistors are used as switching elements or the like. The thin film transistors are formed so that island-shaped semiconductor films are formed over a substrate having an insulating surface by a CVD method, a photolithography process, or the like, and parts of the island-shaped semiconductor films are used as channel forming regions (for example, Reference 1: Japanese Published Patent Application No. H08-018055)
FIGS. 19A to 19C are schematic views of a general thin film transistor. First, a thin film transistor has an island-shaped semiconductor film 903 formed over a substrate 901 with an insulating film 902 serving as a base film interposed therebetween, and a conductive film 905 serving as a gate electrode with a gate insulating film 904 interposed therebetween. The conductive film 905 is provided so as to cross the island-shaped semiconductor film 903. The semiconductor film 903 has a channel forming region 903a formed in a region which overlaps with the conductive film 905, and impurity regions 903b forming source and drain regions. Further, a conductive film 907 forming source and drain electrodes is provided so as to be electrically connected to the impurity region 903b. FIGS. 19B and 19C show cross-sectional structures taken along the lines C1-D1 and C2-D2, respectively, in FIG. 19A.